Generally, in an optical information recording medium capable of writing in optical information, a synchronization signal for searching position and address information (hereinafter, such information is called “pre-format information”) is included in a disc substrate in the form of a phase groove. Such pre-format information can be in the form of a zigzag line (wobbling the groove), or can be represented by changing the length, distance, and position of a discontinuous groove (hereinafter, such groove is called “phase pit”).
For the purpose of increasing the recording capacity of an optical disc, it is desirable to reduce the distance between the grooves employed as the information recording track (hereinafter, such distance is called “track pitch”). However, the need for sufficient C/N restricts the recording capacity of the optical information recording medium in the case of utilizing the wobbling method.
The published specification of Japanese Laid-open Patent Publication No. 9-17029/1997 proposes a method of forming the phase pit on the lands between the information grooves. FIGS. 11A–11C illustrate such an optical information recording medium, where a phase pit P is formed on the land L between the grooves G. A phase P resembles the run of a ladder, namely, connecting the grooves G of two adjacent information tracks.
Such phase pits P, can be read with photodiodes split into two in the radius direction of the optical disc (the direction perpendicular to the track direction) in a light-receiving system, and by detecting the signal obtained through optoelectric conversion by the photodiodes. For more detail, refer to FIG. 8 and the explanation corresponding to FIG. 8 in the published specification of Japanese Laid-open Patent Publication No. 9-17029/1997.
In the case of phase pits P present on both lands L at the right and left sides of a groove G, the pre-format information is simultaneously read out and can cause “cross-talk”. In order to reduce cross-talk, two types of pre-format information phase pits P are formed—for the even number EVEN and for the odd number ODD, and those patterns are changed from one to the other in case of cross-talk situations. For more detail, refer to FIG. 2 and the explanation corresponding to FIG. 2 in the published specification of Japanese Laid-open Patent Publication No. 9-17029/1997. By adopting the above-mentioned method, cross-talk can be reduced.
However, it is technically difficult to determine ahead of time where cross-talk would occur, that is, the position where phase pits P simultaneously exist on the lands L at the right and left sides of a groove G when exposing a master, so as to change from one to the other of the EVEN pattern for the even number and the ODD pattern for the odd number. If there is no error in keeping track of the revolutions of the master being exposed, the position for the cross-talk occurrence can be obtained by calculation and the phase pit pattern of the pre-format information of the change-over between the EVEN pattern and ODD pattern can be encoded. However, there can be an error (in general, not larger than 0.1%) that can introduce inaccuracies in this method of calculation.
In practice, an additional factor that makes it difficult to maintain accuracy is that the length of the phase pit P in the track direction is of the order of sub-micron, making it necessary to monitor the rotation of the master during exposure to nanosecond (ns) accuracy.
There exists a method of employing a push-pull signal (push-pull signal=differential signal) to read (reproduce) pre-format information formed with phase pits. The reproducing principle thereof is described referring to FIGS. 12A–13B. FIGS. 12A and 12B illustrate the waveform of a push-pull signal generated in the vicinity of a ladder-type phase pit P when the reproducing beam B traverses in the radius direction of the disc. The push-pull signal becomes a sinusoidal wave having a period equal to the track pitch TP. In the ladder-type portion, where the phase pit P exists, the shape of the cross section in the radius direction is asymmetrical at the sides of the track center shown by the dotted line. The center of the phase pit P shown by a dot-and-dash line in FIG. 12A is effectively shifted by a distance s in the radius direction from the track center of the groove G.
In the case of tracking along the groove G as shown in FIG. 13A, a peak of amplitude A occurs in the push-pull signal at the position of the phase pit P as shown in FIG. 13B. Therefore, if the presence or absence of the peak as shown by A is detected, the pre-format information formed by the phase pit P can be reproduced. FIG. 14 shows an example of the phase pit P that can be detected in this manner.
Where two phase pits P flank a groove G along the same radius, that is, the cross section position of the tracks Tr3 and Tr4 shown in FIG. 11B (refer to FIG. 15A), the shape of the cross section in the radius direction of the ladder-type portion does not become asymmetric, namely, no positional shift occurs at the center of the phase pit, as in apparent from the push-pull signal shown in FIG. 15B. The peak A does not appear in the push-pull signal in the case of reproducing the signal by performing the tracking control along the groove G in this case. Namely, in case the phase pits P exist at the same time on the lands L situated at the right and left sides of the groove G, there arises a problem that the pre-format information formed with the phase pit P cannot be detected reliably. Consequently, in order to solve the above-mentioned problem, even in the case of reproducing with a push-pull signal, it is necessary to prepare two types of pattern—EVEN for the even number and ODD for the odd number, of the pre-format information formed with the phase pits P and change over those patterns and use one of the patterns in the case of an arrangement generating the cross-talk.
Furthermore, when information is recorded in the groove G, the recording mark M can spread in the radius direction, through the delineation between the groove G and the land L. When recording is done in the vicinity of the phase pit portion formed as shown in the aforementioned published specification, the recording mark M can spread into the phase pit P formed on the land L as shown in FIGS. 16B–16D. Such protruding of the mark M can degrade the phase pit signal. In the case of FIGS. 16B through 16D the phase pit signal can be distorted and its signal amplitude lowered. As a result, the phase pit single P may not be detected reliably at the time of reproducing, and thereby the address information may not be reproduced.
One solution for detecting pre-format information formed with phase pits P even in case that the phase pits P exist at the same time on the lands L situated at the right and left sides of the groove G, is disclosed in the published specification of Japanese Laid-open Patent Publication No. 9-230696/1997. FIGS. 17A–17C show this. In FIG. 17A, the phase pit P is asymmetric in the radius direction. Namely, the phase pit P is not formed completely in the ladder-type shape and the length of the phase pit P is shorter than that of the tract pitch.
FIGS. 18A and 18B show an example of the signal waveform from the phase pit P. According to the proposed example, the optical information recording medium is constructed such that, even though the phase pit exists in the adjacent track(s), the track center of the phase pit and that of the groove G necessarily shifts by a distance s form each other. In such structure, the phase pit can be detected more reliably. However, even in the proposed case, the spreading of the above-mentioned recording mark M at the time of recording can cause problems.
Accordingly, this patent specification is directed to realizing an optical information recording medium which is not affected by cross-talk even when phase pits exist on the lands situated at the right and left sides of a groove and in which the address information, etc., encoded by phase pits can be reproduced reliably.